1. Field of the Invention
The present invention relates to a developing device configured to develop an electrostatic image with a developer including a toner and a magnetic carrier. In addition, the present invention also relates to a process unit and an image forming apparatus using the developing device.
2. Discussion of the Background
Developing devices, which develop an electrostatic image formed on an image bearing member such as photoreceptors using a developer including a toner and a magnetic carrier to form a visual image, have been used for image forming apparatuses such as copiers, facsimiles and printers. In such developing devices, the developer is fed to a developing region, at which a developer bearing member (such as developing rollers) faces the image bearing member, while borne on the developer bearing member, and the toner present on the surface of the magnetic carrier in the developer is attracted by an electrostatic image on the image bearing member, resulting in formation of a toner image thereon. The developer (magnetic carrier) used for development is returned from the developer bearing member to a developer containing portion of the developing device to be reused. Since the toner is thus consumed, the concentration of toner in the developer contained in the developing devices gradually decreases. Therefore, it is typically performed that the concentration of toner in the developer is detected by a sensor, and a new toner (i.e., a fresh toner) is properly supplied to the developing devices to control the toner concentration so as to fall in a predetermined range.
Such a sensor is used not only for detecting the concentration of toner in a developer contained in a developing device, but also for determining whether a fresh developer (hereinafter sometimes referred to as an initial developer) is properly set in the developing devices. For example, when the developer in an image forming apparatus is replaced with an initial developer, a new cartridge, which has an initial developer container in which the initial developer is contained while sealed to prevent occurrence of developer scattering, is typically used. A user sets the new cartridge in the image forming apparatus and removes the seal to feed the initial developer to an agitating section of the developing device. In this regard, if the initial developer is not well fed to the agitating section (for example, due to unsealing), a problem in that the sensor judges that the toner concentration is low, and thereby a fresh toner is continuously fed to the developing device may occur. In order to prevent occurrence of such a problem, the sensor is also used for determining whether an initial developer is properly set in the developing device.
Such toner concentration sensors are typically sensors which detect the concentration of toner in a developer by measuring the magnetic permeability of the developer and output a voltage depending on the magnetic permeability. Specifically, when the concentration of toner in a developer changes, the magnetic permeability of the developer changes. Therefore, the concentration of toner in the developer can be determined by measuring the magnetic permeability of the developer. In recent years, various high sensitive sensors which can measure magnetic permeability with high accuracy have been proposed and/or developed.
However, the present inventors discover that when such a high sensitive sensor is used, a problem in that the advantage thereof cannot be well used or a problem in that the toner concentration is mistakenly determined often occurs. Specifically, sensors having a relatively low sensitivity (i.e., the rate of change of the output voltage from the sensors is low against change of the magnetic permeability of a material (e.g., developer) to be measured) have a property such that variation of sensitivity among the same sensors is relatively small (i.e., have a small individual variation in sensitivity). Therefore, the control parameter (such as shift in output voltage against change of toner concentration of 1%) can be set to one preset value even when two or more of the same sensor are used. In contrast, high sensitive sensors have an advantage of being capable of measuring the toner concentration with high accuracy but have a drawback in that variation in sensitivity is relatively large when two or more of the same sensor are used. When such high sensitive sensors are used, the control parameter has to be set to the intermediate value of the range within which the control parameter of the sensors changes. Therefore, a problem in that the control parameter shifts from the proper value for a sensor depending on the property of the sensor can occur. In this case, the advantage of the high sensitive sensors cannot be used.
As a result of the present inventors' study, we found that it is difficult for high sensitive sensors to determine whether an initial developer is present in a developing device while measuring the toner concentration with high accuracy. Specifically, toner concentration sensors such as low sensitive sensors and high sensitive sensors output a voltage by changing the voltage input thereto depending on the magnetic permeability of the developer. In this regard, the level of the output voltage largely changes depending on choice of sensor even when the same kinds of sensors are used. For example, there is a case in which when the magnetic permeability of the same developer is measured with two of the same sensors while the same voltage is input to the sensors, one of the same sensors outputs a voltage of 2.5V but the other sensor outputs a voltage of 2.9V. In this case, the toner concentration of the developer cannot be accurately measured with the sensor. In order to prevent occurrence of such a problem, an initial input voltage correction operation such that when an image forming apparatus starts to be used or a developing device of the image forming apparatus is replaced with a new developing device, the voltage input to the toner concentration sensor thereof is changed so that the output voltage of the sensor becomes equal to the predetermined voltage is typically performed. By performing this correction operation, the output level of the sensor can be adjusted even when the sensor has a large variation in sensitivity. However, the operation of determining whether or not an initial developer is properly set has to be performed before the initial input voltage correction operation. Therefore, the output level variation problem of the sensor is not solved at this stage.
Conventional low sensitivity toner concentration sensors have such a relatively small variation in output voltage as to be able to determine whether or not the initial developer is properly set. Specifically, when a low sensitivity toner concentration sensor is used, it can be determined by low sensitivity toner concentration sensors without causing a problem that an initial developer is properly set, if the voltages of the sensors are less than a threshold (for example, 0.5V). However, the present inventors discover that high sensitive sensors cannot have such a threshold when considering the variation thereof. Namely, when a threshold is set for a high sensitive sensor, a problem in that it is mistakenly determined by the sensor that an initial developer is properly set even if the initial developer is not set in reality, or vice versa occurs.
Because of these reasons, a need exists for a developing device which can properly determine the toner concentration while properly determining whether or not an initial developer is set even when a (high sensitive) sensor having relatively large variation is used.